The present invention relates to an arrangement of a spark plug in a cylinder head of a single overhead camshaft engine having two intake valves and two exhaust valves at each cylinder.
In such a four-valve type engine, it is preferable to dispose a spark plug at a center of a square formed by quadrangularly positioned valves for the sake of improvement of the ignition of an air-fuel mixture and flame propagation. However, in the single overhead camshaft engine, a camshaft is disposed in a vertical plane including the center of the square so that it is difficult to position the spark plug at the center.
Japanese Patent Application Laid-Open 60-187706 discloses a single overhead camshaft engine having four valves at each cylinder wherein a spark plug is obliquely disposed between intake-valve rocker arms so that a center electrode of the spark plug is placed in the center of the combustion chamber. However, a large space is necessary between the intake valves to dispose the spark plug. As a result, the inclination angle of the valves inevitably increase, so that the engine cannot be made compact. On the other hand, in the case of a horizontal opposed type engine, since the axis of an elongated hole in which disposing the spark plug is disposed is parallel to the intake valves, it is difficult to change the spark plug for a new one.
In another arrangement disclosed in Japanese Patent Application Laid-Open 61-138862, a spark plug is obliquely disposed between two exhaust ports. In such an arrangement, the spark plug is subjected to heat the exhaust ports. In order to protect the spark plugs from the exhaust heat, coolant passages are provided between the spark plug and the exhaust ports. Since the coolant passages occupy a space, the engine becomes large in size. On the other hand, in case of the horizontal opposed type engine, since a hole in which the spark plug is disposed opens at a lower side of the engine, it is impossible to change the spark plug for a new one.